The present invention relates to a printing device which can print characters, and which can accentuate at least part of the outline portion of the characters for reasons of aesthetics and clarity.
A dot-matrix printer prints characters in accordance with the character data stored in a specified portion of a memory, generally known as the "character generator." The character data represents the dot-patterns of the characters to be printed. To print a character, its dot-pattern is read out from the character generator. Those dot-elements of a printing head which correspond to the dot-pattern are each supplied with an equal amount of energy, thereby driving them and printing the character. To print a character in a dot-pattern different from that stored in the character generator, the dot-pattern most similar to that of the character in question is read out from the character generator, is altered in accordance with a computer program, and the data representing this new dot-pattern is used for subsequent printing of the character.
This method of printing characters is employed not only in printers which utilize an ink layer having a specific electrical resistance, but also in wire-dot printers and thermal-transfer printers. Generally, the dot-patterns representing alphabetic and Chinese characters are constructed such that any vertical element of the characters (hereinafter called "character element") consists of three columns of dots, and any horizontal character element consists of one row of dots. Hence, when printed, the horizontal character element appears less dark than the vertical character element. Further, the printed vertical character element having a width of more than two dots appears dark at the center but somewhat blurred at both outline portions. Consequently, the character, as a whole, appears blurred, not having sharply-defined outlines.
However, if greater energy is applied to those dot elements of the printing head which print dots forming the outlines of the character, than to the other dot elements which print the dots forming other portions of the character the printer can achieve high-quality printing, that is, it can print a character having sharply-defined outlines. In order to accomplish such high-quality printing, some conventional printers have an auxiliary character generator, i.e., another portion of the memory, in addition to the character generator (hereinafter called the "main character generator"). The auxiliary character generator is a memory for storing data items which represent the dot-patterns of the outline portions of frequently used characters. To print any desired character, the two corresponding dot-patterns are read out from the main and auxiliary character generators. An equal amount of energy is applied to each of those dot-elements of the printing head which correspond to the dot-pattern read out from the main character generator, and simultaneously, a predetermined energy is applied to those dot-elements which correspond to the dot pattern read out from the auxiliary character generator. Thus, greater energy is applied to the dot-elements corresponding to the outline portions of the character, than to those corresponding to the other portions thereof, with the result that the outline portions of the character are printed darker than the other portions thereof.
The higher the resolution of the character to be printed by the printer, the greater the amount of memory space the character generator occupies. The greater the number of characters used, the greater the amount of memory space required to store dot-patterns. Under these circumstances, it would not be advisable to use an auxiliary character generator in addition to a main character generator.